The present invention relates to a control system for a variable capacity type compressor for air conditioning systems of vehicles and, more particularly, to a control system for a variable capacity type compressor which is constructed such that the suction pressure is varied by pressure control means.
Conventional variable capacity compressors include a type which comprises a crank chamber, a drive shaft arranged in the crank chamber, a wobble plate fitted on the drive shaft for rotation therewith and axial tilting relative thereto, and pistons coupled to the wobble plate for reciprocating motions in response to rotation of the wobble plate, wherein the wobble plate assumes angularity or angle of inclination determined by pressure within the crank chamber, and a change in the angularity of the wobble plate causes a corresponding change in the reciprocating stroke of the pistons to thereby vary the compressor capacity, as is known, e.g. from U.S. Pat. No. 4,157,233.
In a compressor in general including the type referred to above, the power required for driving the compressor is directly proportional to the ratio of the discharge pressure (i.e. discharge amount) to the suction pressure, and therefore operation at high efficiency can be ensured when the ratio of the discharge pressure to the suction pressure is minimum. To this end, techniques have conventionally been employed in which the compressor is operated at an allowable maximum suction pressure, or at an allowable minimum discharge pressure or discharge amount. For example, Japanese Provisional Patent Publication (Kokai) No. 53-107042 discloses a first technique in which an upper limit value of the suction pressure is set independently of the compressor capacity, and the compressor capacity is increased to lower the suction pressure when the suction pressure exceeds the set upper limit value due to an increase in thermal load on the air conditioning system or due to other causes. Further, Japanese Provisional Utility Model Publication (Kokai) 59-182059 discloses a second technique in which an allowable, predetermined minimum discharge amount (i.e., minimum capacity) is set, and measures can be taken to stop the compressor when a situation that the capacity falls below the minimum capacity is brought about due to decrease in the thermal load, or due to other causes.
In the above-described first technique in which the upper limit value of the suction pressure is merely set, no irrationality would occur if the thermal load varies to a smaller value or decreases. If the thermal load varies to a larger value or increases, however, irrationality would occur that since the compressor capacity is increased when the suction pressure exceeds the upper limit value, the compressor cannot operate at a value equal to or lower than the increased capacity, in spite of the fact that the increased capacity is too large for the increased thermal load.
The above-described second technique, which interrupts the operation of the compressor if only the capacity is at the value equal to or lower than the allowable minimum capacity, has the disadvantage that as the operation in which the minimum capacity is maintained continues for a long time, the recirculation of lubricating oil within the compressor would be deteriorated, and this would tend to cause so-called seizure. Moreover, since it is necessary to increase pressure within the crank chamber so as to keep the angularity of the wobble plate at the minimum, there would be a fear that the compressor is damaged by the long-time increase in the crank chamber pressure.
As described above, in either of the conventional techniques, the operation of the variable capacity type compressor is based on whether or not there is a variation of an operating parameter with respect to a single set value thereof. Therefore, no measures are taken in compliance with the vehicle running conditions such as the thermal load, the engine operating condition and the like, and no sufficient consideration is made to the compressor operation at high efficiency and to the compressor operation over a long service life.